Build the $20 Utterpower Interlocking Transfer Switch.


Cool looking old switch I found in the bowels of a Seatle Building.

No doubt, there’s a lot of us that build less than ideal stuff for our own use. It’s because we are thrifty and think we can get away with things.. sometimes we don’t 🙂

Utterpower 20dollar-Trans-Front

Utterpower 20dollar-Trans-Front


 Fact is, when the lights go out, we are sometimes under pressure and in a hurry. This is when a lot of stuff gets burned up, and many people will tell you.. “sure I knew better”, but I just forgot to open that breaker before I did this or that.  Since your wife claims to own a good number of appliances,  you best think about reducing the risk of burning HER stuff up; cause you know you’ll never hear the end of it, and your meals could be pretty basic for a while.   

I talked about how inexpensive it is to buy a Kill A Watt, and how everyone who owns one should use it to monitor frequency and voltage, but there’s something just as important that we should all consider… an ‘interlocking transfer switch!’

This simple to make device will allow only one ‘source’ to power your appliances at a time, and it will assure you don’t make smoke, ruin a generator, or electrocute the man on the power pole who is attempting to restore your power.

You might read my article ‘How to install an generator transfer switch at Zach’s house first. This will give you an idea how to do it best.  The way I show here is far better than nothing, and if you want to run more than one circuit like the loop of wall plugs in the family room, you could find a box with four positions, and cut two notches in the interlock.. Hey, you’re a DIYer right? Once you have the idea, you can roll your own, and make it the way you need it..

20 Dollar Utterpower Gen Trans Wiring

20 Dollar Utterpower Gen Trans Wiring

 If you are wiring this in permanent, the lead marked grid will be connected to the output of your circuit breaker in the main box.  The lead marked load will be tied to the wire in the circuit box that feeds the circuit you want to power with the generator, and of course, the Generator lead goes back to the generator, or the box that contains the plug in where you will cross connect your portable power when you need it.

For $20, can you afford to keep  running your extension cord through the window to a junction box, and then attempt to run your important stuff off other extension cords from there? Are you tired of looking for the cords in the dark when you need them?

If $20 is too much, there’s another trick I used for years  that cost me two dollars that made my life a little easier. It does require that you remember to open a breaker!  I went to a plug in the family room that shared a wall with the garage. I opened the wall plug and proved that only wall board was between the back of that plastic plug box, and the garage. Once I was satisfied things were ok, I pushed a sharp piece of wire thru the wall (a piece of coat hanger) to locate the spot.   I then went just below that box to assure I had adequate room for what they call an ‘old work box’.  This is found in near any hardware store,  a plastic  box that has wings that fold out behind the sheet rock to secure it instead of mounting the box to a stud as new construction boxes  do. 

Hold the box in place, use a pencil to trace around it, and then carefull cut the hole for the box.. do not crowd the wall stud! Once you have your box in, you can wire in a recessed male plug, you should consider having a cover over it if you have curious kids, use your good judgement. Some people make a back to back male to male plug and store it with their generator.. (this is also risky and you need think when you use it). The advantage? you keep the generator outside the garage door, the extension cord doesn’t have to get slammed in the door, or go through an open window, the carbon monoxide stays outside, you stay inside.

The DIYer Transfer switch might even be legal in your area if the breakers are ‘switch rated’ you can read more about that in the other article  mention above.

Facts are, we are likely to see more power outages in our future, so why not get ready for it? You don’t need to invest much, and many times a small generator will run that blower in your natural gas furnace and allow you to stay warm, watch TV, run the microwave, and wonder what the people who didn’t plan at all are doing? Maybe you invite your good neighbors over for a game of cards?        

 Remember.. you will always check with your city, county, state, and Federal authorities, I have no license to tell you a thing. This article is all about things you might investigate doing..

the parts I used? A box and two breakers from Home depot, I like Square D Homeline as they seem good and are cheaper for me to buy. I also used a piece of alum sheet metal, also found at Home depot, and a few small screw and nuts from the junk drawer.

Yes, I did cheat on the slots, but it your careful you can do as good with steel and a 1/4 die grinder with a cut off wheel, just drill the holes at the ends first, and cut to them using a guide. And you can make your template out of paper, and then glue it to your work one you have the slots proven on the front of your box.

How about a bit of trivia to end this post? All Kill A Watts I’ve ever seen have a miinor error.. when you have a real solid 60.00 HZ, they read 99.9. when you see 99.9 there’s a very good chance you’re looking at commercial power. If you or a friend has a lapse, and wonders what you’re running of of.. a steady 99.9 is a pretty strong clue..       


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10 Responses to Build the $20 Utterpower Interlocking Transfer Switch.

  1. curlie says:

    I mentioned in my last post, a comment on Installing a gen at Zack’s house, what a good article it was. This $20 interlock is a great idea BUT, in my research prior to installing an interlock-type transfer, I found that there would be code objections to altering the box face plate by drilling holes and installing the metal plate in some, perhaps many, locales. One advantage of this $20 interlock seems to be that you can choose any breaker/ breakers in your main panel that is rated equally with your interlock breaker set, 30 A. That gives lots of flexibility in an extended outage, where you may want to wash clothes after 4 or 5 days of sweating after that August hurricane. A cold water wash should be capable if you just turn on the washer and the water well. Remember, there’s a chance that different appliances will start simultaneously, so do not over-tax the genset by trying to run lots at one time.
    One alternative to making your own, you can buy one that fits your main panel and it allows you choose any circuit or two or three or more in that panel. Again, remember not to over-tax your gen. A few disadvantages to this interlock sold for many panels is the cost at just over $100 for a simple sliding or rocker-type metal blocker plate; it requires altering the panel face to install it (may negate UL approval of that box), and third, it takes up LOTS of space. This last concern was the clincher for me; I simply did not have eight spaces to dedicate in my panel for this either/or – genset/poco juicer.
    So, what did I do? I was really tired of the extension cords coming through the windows from the gen on the carport and worried that my late-eighties aged parents would trip over one of the many it took for flexibility. We lived like this for two weeks after Hurricane Katrina and I was determined to do something to improve generator power.
    I found that Murray also made a mechanical interlock system that worked in a small (12 space) box using Murray breakers. If I remember correctly, the box, the interlock, and the breakers were around $120. One disadvantage: like many other transfer switches, I had to choose ahead of time which circuits to move to the Murray panel. The Murray panel was mounted under the carport and just through the wall from my breaker panel in my kitchen. This made moving the “chosen circuits” easy work and out of the weather. I knew from prior experience which circuits I would use daily and figured I could always run an extension cord through the window for an hour if I wanted to run the washing machine during an extended outage. One samll advantage here – I created space in my crowded inside panel.
    How’d it work out? Extremely well for my tastes. We lost power for Huricane Isaac, albeit for only a couple of days. My well-maintained 5500W genset was hooked up, gassed up, cranked up and up to the task. No cords were needed during that period of time and we had well water, satellite TV, a few lights, a running frig, one air-conditioned room, and fresh coffee dripped in the morning! Maybe best of all – my elderly parental evacuees from New Orleans remained upright when walking about the house. My spouse and I are campers from way back and do not feel the need or want to install the whole-house auto generator that costs a fortune to install and fuel on a daily basis, but a few conveniences for daily living and the ability to stay abreast of the breaking news is golden!
    I might mention that my new Murray panel was near the front of the carport and I keep my genset near the rear, some 30 ft away. Rather than put a generator inlet beneath that panel, I installed it on the side of the house near the back of the carport and ran heavy wiring beneath the house (raised floor, enclosed crawl space) then back out into conduit just below the Murray panel. I also puchased a real generator cord with a shielded (female) end to plug into the generator in box, allowing me to keep the gen set 20 ft away from the house (fumes, noise). This added to the cost of the project, but was well worth it. Everything was staged and ready for the impending outage. One thing about hurricanes you do get some fore-warning.
    The point of my post – you do have alternatives DIYers.

    • George B. says:


      Hands on experience is what our pages are all about. Your note about older parents and tripping hazzards are a great reminder to us all. >We< are all getting older, and now is a good time to remove those tripping hazzards for ourselves as well. Falling down at sixty plus is way different than it was falling at twenty years old. Darkness and cords running here and there create liaibility, so we might attempt to access which risk is greater, we need do that in the same way you did.. taking time to study the options, and choosing one best for our skills and pocket book, but we best know that our pocket book is evaporating as I write. The printing press effectively is no more than a direct withdraw from your savings account you took pride in building.

      Your mention of the cord is a good one, there are plenty of junk 16 gauge cords out there, and marketeers even build them with thick fat insulation attempting to fool the consumer.

  2. Butch says:

    George, as far as I know the breaker interlock is legal anywhere as kits are factory made and UL listed but never confuse me with being a ‘lectric expert. “Always check local codes” is wise advise..
    Recently helped a fellow out with an emergency generator install and being pressed for time I was going to purchase a kit instead of fabbing one,, $115,, and to plan B we went and fabbed one up. A tip for those with mandatory electrical inspections is to be sure the homemade interlock looks nice and is well installed. Emergency use is a differant animal than off grid and typicaly a 220 feed from the generator is the way to go for simplicity of both usage and installation. A person is wise to use the top breaker as a generator feed so the lock out plate is an easy make and install. The factory plates I am aware of will require the use of the top breaker in the box. As for the recessed male recepticle you spoke about (please dont use a “meet Jesus” cord!) a person will find that box and recpticle to be a rather expensive item if purchased new if he is dealing with very many amps and 220 volts. A tip is to find somebody who is wrecking RVs. They have a nice weather proof power inlet box and extension cord.

    • George B. says:

      All good advice Butch.. As I said, many of us have used devices that take thought and care, and that back to back male plug is something we find in junk draws or stored with small generators, best we know what it was used for.. to power a single circuit of wall plugs after the breaker has been opened. I mention it as something people might find stored with a generator, it is dangerous, but perhaps safer than bringing the generator in doors? Yes, we do read about people doing that every year.. somewhere.

  3. George B. says:

    Butch, (my opinion)

    The 240 VS 120 VAC decision has everything to do with the loads in the house. If you read about ‘Zach’s House’, they have an ideal situation for 120VAC only.. There is NOTHING in the house that runs on 240 they need, and the 120 VAC only option removes the problem of loading one side of the stator too heavy in a small generator, this is a very typical problem for a lot of DIYers. They often have no idea they’ve over loaded one side of their generator and it will NOT cover their intended use durign an outage.

  4. Tracy says:

    We have so many power outages that I am beginning to think it will just be easier to go back to being 100% offgrid. In fact, today is a beautiful day with not a cloud in the sky nor a breath of wind, yet we have already had one short power outage today.
    That being the case, one instance of using extension cords (for the most part of a week, that time) convinced me that I do not want to go that route again. For awhile I backfed through the clothes dryer circuit, but there were too many connections in that path. Too many opportunities for accidents, and too much voltage drop. So I finally bit the bullet and did some minor rewiring of my main junction box to add a dedicated generator circuit with 6 gauge wire and a 30 amp breaker. That adds up to 7.2KW. Since my generator is rated 7.5KW continuous, any overload should trip the breaker before it can overload the genny.
    My breaker panel has the main breaker mounted horizontally at the top center and the rest of the breakers in two vertical columns, also with the individual breakers mounted horizontally. I moved some of the existing circuits around a bit so I could locate the generator breaker in the top slot of the right hand column. That makes it simple to add a sliding interlock to prevent the main and generator breakers being energized simultaneously.

    • George B. says:

      Hi Tracy,

      I always enjoy your comments.

      If we take time to get the big picture, we will anticipate far more outages. The grid was designed to run on reliable power, and there are even typical penalities a power provider must pay when he defaults on the contract. This is done because another entity must quickly scramble to help meet the real time demand, when the winds dies, or the clouds move in.. these are peak hours, and paid at preium prices when others need scramble to cover your failure to produce. All this normally does not work well for wind or solar, and those who play the green game for cash want the money, and none of the responsibility that comes with it.. doesn’t that sound all too familiar ??

      Meanwhile, Las Vegas never did ask if all that Amonix stuff worked, and now Harry Reid’s Son is pushing more Green for his own profit. Nevada PUC is not so easy, but the average person in Las Vegas? I think you could piss on their back and tell them it was raining and they’d never know the difference. go read the Las Vegas Sun.. not one comment from the peanut gallery was ever asked.. at least none I could find..

      And Amonix? Well, there’s some workers full time at Hatch right now, they’re attempting to rehab a plant that is performing badly. My prediction? Thsi winter will make a mess of all their work, this Amonix stuff is best run in doors, where it’s protected from the weather.

      The greenies think the answer is to turn off your appliances when they fail to produce, this is called the smart grid.

      The truth about Green? when you do it, it’s green, when they do it, it’s not..

  5. Jeff Maier says:

    I believe I have read that when Intermittant sources of power such as wind and solar get to around 10 to 20% of grid capacity it starts becoming disruptive. So if you have 3KW of solar panels on your roof, whether they are producing or not they will have no impact in grid stability. However, if you have a Columbia Gorge scale farm at full output during a period of low demand, a lot of that power can’t be used or you get voltage and / frequency disruptions. On the other hand, during periods of no wind when demand is high, expensive natural gas peaking plants equal in capacity to the wind farm must be fired up, and fired up at a moments notice. Even maybe kept running as a spinning reserve. So, IMHO, none of thes technologies on a utility scale will be viable until some cheap, efficient, utility scale storage becomes available. Liquid metal batteries perhaps? Maybe an incentive which pays the producer a premium to produce a constant output 24/7?

    • George B. says:

      Jeff, I think it’s pretty simple, the rules of the game have been worked out over many years, if you want to play, you need contract with an entity that will pick up where you left off if the clouds come, or the wind stops. the name of the game is to make energy when it’s needed. unwanted commodities normally sell for a very low price. But the sharks who are playing in the green pool want the higher price. (at our expense).

      It’s not so much a 24 hour thing, it’s more about producing when the power is needed. Like on Labor day, when everyone is camping. I’d bet usage is down!

      So, you’re not hungry? How’d you like being forced to buy the meal anyway?

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